Feature specification of #3090, allowing an inline class to implement its representation type

[eernstg]

This PR contains a proposal for an addendum to the feature specification of inline classes, specifying how we could support the situation where an inline class V has an implements ... R clause where R is the representation type (or any supertype thereof).

The point is that this provides (1) the subtype relationship V <: R, and (2) a receiver of type V can have members of R invoked on it.

In this proposal, V is allowed to redeclare members of R (which means that V "overrides" the implementation of that member, albeit with static dispatch only).

Conflict resolution is specified to be the same as it has always been for superinterfaces: If both R and some other superinterface have a member with the same name then there is an error, but V can redeclare the member and that will resolve the conflict.

[eernstg]

Note that the proposal in this PR is included in the proposal about extension types, #3182.

[eernstg]

Closing: This feature has been included in the 'extension types' feature specification, so there's no need to have a separate feature specification (which is what this PR will add).